Electrically heated personal comfort or medical aid devices typically include an electrical heating means, such as a resistance heater wire, disposed between a pair of fabric covers. Heat is generated and supplied to the user when electrical energy or current flows through the heater wire. To guard against overheating a manually operated control is included to open the circuit providing electrical energy to the heater wire. However, it is also desirable to have a self monitoring temperature sensitive device that will control the flow of electrical current to the electrical heating means in the event that the user does not detect an overheating condition. Such a condition may result from the placement of a thermally insulating cover over the heating device, a bunching of the blanket, irregularities in the amount of electrical energy applied, or an electrical or mechanical malfunction. The overheating may be detrimental to the user and the materials adjacent to the heater wire. Thus, electrically heated covers typically include apparatus to sense an overheating condition and reduce or interrupt or otherwise control the flow of electrical current to the electrical heating means.
Typically, electric blankets, hot pads, heated socks and the like experience a degree of flexing, folding, bending and crumpling. Thus, the electrical heating means and electrical temperature sensor must also be capable of bending to accomodate such flexing. Metallic wire may permit some flexing or bending, but an excess will cause the metal to fatigue and break, resulting in an undesirable open circuit. Notwithstanding, most overheating detection circuits have utilized an electrical sensor having one or more metal conductors.
Two conductor prior art sensing wire is disclosed in U.S. Pat. Nos. 2,745,943, issued May 15, 1956, and 3,222,497, issued Dec. 7, 1965. Each has a central core surrounded by a helix of a first metallic electrical conductor. A temperature sensitive impedance material coaxially jackets the first conductor and central core, and a second metallic conductor is wrapped around the outer surface of the impedance material. An electrically insulating jacket coaxially encases the entire sensor wire. The resistivity of the impedance material varies inversely with temperature in a predetermined relationship, and is commonly referred to as NTC (negative temperature coefficient) material. Thus, any overheating may be detected by monitoring current flow therethrough. Such a sensor wire, although functional, requires multiple metallic conductors, and expensive raw materials and manufacturing equipment. It further requires a multistep and time consuming manufacturing process, including several wire wrap operations, which further adds to the cost thereof.
Another type of two wire sensor for use in thermostatless overheat protection circuits uses temperature sensitive dielectrics. Commonly assigned U.S. Pat. No. 3,683,151 issued Aug. 8, 1972, disclosed a sensor having a temperature sensitive dielectric between a pair of signal wires. A single gate controlled semiconductor switch responsive to the sensor reduces the energy delivered to a heating element in response to an overheating condition. Commonly assigned U.S. Pat. No. 4,315,141 issued Feb. 9, 1982 disclosed a sensor having a pair of conductors normally separated by a temperature sensitive dielectric. A pair of solid state switching devices are in series with the heating element circuit, and the gates thereof are electrically connected to the sensor. The switching devices are rendered nonconductive in response to an overheating condition. These circuits utilized relatively expensive sensors and control circuits, and included additional wires not found in the present invention, which may reduce their operational lifetime.
Other patents, such as U.S. Pat. No. 3,410,984 issued Nov. 12, 1968, disclosed electrically heated bedcovers with the heater wires themselves formed by a pair of resistance conductors separated by a layer of material having a positive temperature coefficient (PTC) of resistance. As the temperature of the PTC material increased, it expanded, increasing the impedance thereof, reducing current flow between the separated conductors, and reducing heat output. It did not include a discrete electrical sensor.